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Ultrafast Charge Dynamics in Dispersions of Monolayer MoS2

Nanosheets

Georgia Kime1, Marina A Leontiadou1, Jack R Brent3, Nicky Savjani2, Paul O'Brien2,3

and David Binks1*

1 Photon Science Institute & School of Physics and Astronomy, University ofManchester, M13 9PL, UK2 School of Chemistry, University of Manchester, M13 9PL, UK3 School of Materials, University of Manchester, M13 9PL, UK*To whom correspondence should be addressed

ABSTRACT Ultrafast charge dynamics in dispersions of MoS2 nanosheets in N-methyl-2-pyrrolidone are reported. Samples were prepared by the ultrasonication-assisted exfoliation of MoS2 powder, resulting in nanosheets that were predominantly monolayer and had average sheet size of 32.4 ±0.1 nm. These dispersions were characterized using absorption and photoluminescence spectroscopy, transient photoluminescence measurements and atomic force microscopy before the ultrafast charge dynamics were studied via transient absorption spectroscopy. The transient absorption spectra exhibited bleach peaks and photo-induced absorption peaks in spectral regions corresponding to both the A and B excitons of MoS2. The growth and decay of the features in the B exciton region were determined largely by the dynamics of the exciton population, whilst the features in the A exciton region depend on the dynamics of both excitons and trions.

Email: [email protected]

1. INTRODUCTIONAtomically-thin sheets of transition

metal dichalcogenides (TMDs) have emerged as a promising new class of materials that have properties distinct from their bulk counterparts1,2. In molybdenum disulphide (MoS2) for instance, the optical gap increases from 1.2 eV to 1.9 eV and becomes direct as the thickness reduces to a monolayer, making it of interest for applications involving optical absorption and/or emission in the visible region3. This, coupled with the excellent catalytic activity exhibited by MoS2 and its large redox potential, makes it a promising material for photocatalytic processes such as the hydrogen evolution reaction (HER)4–10 and CO2 photoconversion11–13.

The performance of TMD nanosheets for many applications, including photo-catalysis, depends on the dynamics of photo-generated charges. Processes that occur on sub-nanosecond time-scales often determine these dynamics, which has motivated a number of ultrafast optical studies. However, these works have to date

largely focused on TMD samples formed via mechanical exfoliation14–18 or chemical vapor deposition (CVD)19–21. Preparation of TMD nanosheets by ultrasonication in a suitable solvent is a cost-effective and scalable fabrication method, well-suited to the production of large quantities of material22–26, and the liquid-dispersed nanosheets produced by this method can be used directly for photocatalysis8,27. Nanosheets in this form can also be further processed via inkjet printing28,29, spray-coating30 and doctor blading31 to fabricate thin-film heterostructure devices such as batteries, sensors and solar cells.

Defects6–8,25,32–39 and adsorbates1,19,40–42

play an important role in determining the charge dynamics in TMD nanosheets. The high catalytic activity of 2D MoS2 is attributed to the large density of edge defects behaving as catalytically active sites6–8,13,25,35,36 and the sensitivity of the electronic properties of monolayer MoS2 to the presence of adsorbates, which has led to its application in chemical sensing43. A high defect density can exhibit itself in other ways; recent work has shown that non-

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radiative defect-related recombination is dominant in many MoS2 samples, promoting sub-picosecond charge trapping to mid-gap states15,19,37–39 and the ultrafast and efficient dissociation of excitons3,14,42, leading to a typically low photoluminescence quantum yield (PLQY)44. Exciton dissociation can lead to the formation of trions, resulting in absorption and photoluminescence features red-shifted around 30meV from the lowest energy exciton transition40,45. Trion formation can be harnessed in optoelectronic devices as a means of controlling conductivity20 as well as photoluminescence46. Both positive and negative trion formation depends on the Fermi-level and intrinsic doping of TMDs14

and can be encouraged or suppressed via electric back-gating and chemical doping46. The addition of adsorbates has been reported to neutralize n-type doping and prevent trion formation19,47–49 suggesting that the presence and nature of a solvent could alter the generation of trions and other charge dynamics within these materials.

Studies of liquid-phase synthesized TMDs have been presented but the material was 3-6 monolayers in thickness or greater with a lateral size of ~100 nm or more and deposited onto a substrate39,42 rather than dispersed in a liquid, where it might interact with surrounding molecules. Liquid dispersions of few-layer black phosphorus and graphene nanosheets have been studied50,51, as have dispersions of MoS2 and WS2 flakes in a solid3,33. However, to our knowledge, the ultrafast charge properties of MoS2 flakes in liquid solvents remains unstudied. The thickness and lateral extent of the flakes in a dispersion can be expected to have important effects on these charge dynamics. For instance, the exciton binding energy, EX , has been found to depend strongly on layer thickness; it has been calculated to be as much as 0.9 eV for a monolayer and 0.4 eV for a bilayer52, with experimentally measured values for a monolayer reported variously to be 0.44 eV53

and 0.57 eV54. The optical gap for these thicknesses is thus significantly less than the band gap, by an amount equal to EX. Moreover, as the lateral size of a flake decreases, the likelihood of an interaction with an edge state will increase, enhancing the potential for trapping, non-radiative recombination and catalytic activity. Thus, in this work, we investigate the ultrafast exciton

and trion dynamics in liquid-dispersed monolayer MoS2 nanosheets with lateral sizes of a few 10s of nm, reporting the important processes and associated time-scales.

2. MATERIALS AND METHODS.

2.1 Synthesis of MoS2 nanosheets. MoS2 nanosheet dispersions in N-methyl-2-pyrrolidone (NMP) were synthesized using ultrasonication-assisted exfoliation. MoS2

powder (6 µm particle size) and NMP were purchased from Sigma Aldrich. 7.5 mg/ml solutions of MoS2 powder in NMP were ultrasonicated in an Elmasonic p70H benchtop ultrasonic bath (820 W) across 4 horns at 37 kHz frequency and 30% power. Upon completion of exfoliation the samples were centrifuged at 1500 rpm (relative centrifugal force ~180 g) for 45 min to remove unexfoliated material. This process has been described in more detail by Savjani et al. previously 55.

To determine the distributions of nanosheet size and thickness, a small amount of the dispersion was spin coated onto a 300 nm SiO2/Si substrate at 6000 rpm. An atomic force microscope (AFM, Bruker Multimode 8) was then operated in tapping mode using a silicon nitride cantilever tip.

2.2 Absorption and Photoluminescence Spectroscopy. For optical characterization the dispersion was transferred to a 3.5 mL fused quartz optical cuvette with a 10 mm path length, with an optically identical cuvette containing only NMP used as a reference. Steady-state absorption spectroscopy was performed using a Perkin Elmer Lambda 1050 UV/Vis/IR spectrophotometer and resultant spectra were corrected for scattering as detailed in the Supplementary Information. Photoluminescence (PL) spectroscopy was performed using a Horiba Fluorolog-3 spectrofluorometer with excitation at a wavelength of 420 nm. Emission spectra were collected 90° from excitation, and corrected for the wavelength dependence of the lamp and detector.

2.3 Transient Photoluminescence. Photoluminescence decay transients were obtained using the time-correlated single

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photon counting (TCSPC) technique. Excitation at 420 nm was provided by 100 fs pulses from a frequency-doubled, mode-locked Ti:sapphire laser (Spectra-Physics, Mai Tai), the pulse repetition rate of which was reduced from 80MHz to 4MHz using a pulse picker (APE, Pulse Select). Neutral density filters were used to reduce the energy per pulse to 20 pJ. Emission was collected by a series of lenses and, after passing through a monochromator (Horiba, SPEX 1870C), focused onto a microchannel plate (MCP; Hamamatsu R3809U-50). TCSPC electronics from Edinburgh Instruments (T900) were used to produce the decay transients from the resulting MCP signal. To ensure single photon counting, the excitation/emission count rate ratio was kept below 1%.

2.4 Transient Absorption. Ultrafast transient absorption spectroscopy was performed on the MoS2 dispersion using a pump-probe set-up with the sample in a 3.5 mL fused quartz optical cuvette. The experimental setup has been described in detail previously56 with conditions as follows.

Both pump and probe beams were created using a mode-locked Ti:Sapphire laser oscillator (Spectra-Physics, Tsunami)

and a regenerative amplifier (Spectra-Physics, Spitfire) system. The amplifier produces a beam of 1 mJ, 100 fs pulses at a repetition rate 1 kHz and a wavelength of 800 nm. The majority (95%) of this beam was passed through an optical parametric amplifier and harmonic generation system (TOPAS, Light Conversion Ltd.) to produce the pump wavelength of 420 nm. The spot size at the sample was 1.6 mm, yielding pump fluences up to 109 J/cm2.

A mechanical chopper was used to modulate the pump beam, blocking every other pulse. The remaining 5% of the amplifier output was used to produce a white light continuum in a sapphire plate. The resulting broadband beam was split to form probe and reference beams, balanced with a variable neutral density filter in the reference arm. The probe beam passes through the pumped volume of the sample, and then both probe and reference pass through a spectrometer (Princeton Instruments, Acton SpectraPro 2500i) and are detected by two photodiodes. The difference between the two photodiode signals was monitored via a lock-in amplifier synchronized to the chopper, yielding the fractional change in sample transmittance induced in the sample

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Figure 1: (a) AFM height image of MoS2 nanosheets. (b) Absorbance spectra for the MoS2 nanosheets dispersed in NMP (black), and for NMP only (green). Characteristic absorption peaks corresponding to the A and B excitons in MoS 2 area indicated with arrows. (c) Steady state PL response of MoS2 dispersions (black) and NMP (green). (d) PL transients for the MoS2 dispersion obtained for detection at the A (orange) and B (blue) excitonic energies, with transients for NMP only (green) and the instrument response function (IRF, black dotted).








by the pump, ΔTT, which is equivalent to

the differential absorbance, −Δ A , for ΔTT

≪1 (see Supplementary Information).

Finally, this is corrected for the steady-state absorption across the spectrum, giving the differential fractional absorbance change Δ AA.

Sub-picosecond temporal resolution was achieved by using a mechanical delay stage to vary the arrival time of the pump beam at the sample relative to that of the probe beam. Both differential absorbance spectra and decay transients were collected by varying the detection wavelength and pump-probe delay respectively.

3. RESULTS AND DISCUSSION.Figure 1a shows an example AFM

image obtained to determine the nanosheet size and thickness distributions of the MoS2

dispersions. The average thickness of the nanosheets was 0.93±0.01 nm. The interlayer separation for MoS2 has been reported to be in the 0.65 to 1.0 nm range 19,57–60, so this average thickness indicates that the nanosheets are largely monolayers. The optical gap of the nanosheets is thus typically ~1.9 eV, and hence better suited to water-splitting and the photo-reduction of CO2

than the 1.2 eV band gap of many layer or bulk MoS2

because it allows for an overpotential to drive these reactions. The average nanosheet size was 32.4 ±0.1 nm, which is similar to the charge diffusion length in TMDs33. The probability of a photo-generated charge reaching a catalytically-active site at the edge of the nanosheet is thus significant.

Steady state optical characterization demonstrated that the MoS2 dispersion exhibits characteristic absorption peaks at 676 nm and 616 nm as seen in figure 1b. These correspond to the A and B excitons respectively, i.e. direct gap transitions between the spin-split valance band maxima and the conduction band minimum1. A broad absorption peak around 400 nm is also evident, and corresponds to the C exciton, produced by a transition from deeper within the valance band to the conduction band minimum61. Figure 1b also

shows that the NMP solvent makes no significant contribution to the absorbance of the dispersion in this spectral range.

In contrast, steady-state PL obtained from the dispersion originates largely from the NMP, with the shape of the spectrum modified by re-absorption by MoS2, and is consistent with the low PLQY typically reported for atomically-thin MoS2

44. The sharp feature at 479 nm is a Raman peak in NMP62.

A contribution from the MoS2 is evident however, in the PL transients shown in figure 1d. For detection at wavelengths corresponding to the A and B excitons, the dispersion produces a large initial PL component that largely decays within 0.25 ns of excitation, followed by a weaker component that persists until ~0.75 ns. The shoulder at about 0.2 ns that is particularly evident in the decay at the A exciton wavelength is attributed to a combination of the IRF and the growth of PL from the NMP (also shown in figure 1d), which peaks around this time. Emission from the NMP

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Figure 2: (a) Differential absorbance spectrum for a dispersion of MoS2 nanosheets in NMP, at a pump-probe delay of 1.5 ps and an excitation fluence of 68 µJ/cm2. Bleach features, A and B, and photo-induced absorption (PIA) peaks, A* and B* are indicated with arrows. (b) Differential absorbance transients obtained at wavelengths of 606, 640, 669 and 694 nm corresponding to the B, B*, A and A* features, respectively. The inset shows the early part of each transient with dotted lines indicating the rise time of each feature.








also contributes a slow-decaying component to the A and B transients, which remains beyond 1 ns. In comparison, the radiative lifetime for monolayer MoS2 is typically ~10 ns44 so the much shorter PL lifetimes observed here indicate that non-radiative processes largely out-compete radiative recombination, resulting in the very weak emission.

The sub-nanosecond dynamics seen in these transient PL measurements underlines the need to study the processes that affect photo-generated charges with ultrafast spectroscopic techniques as PL is evidently not the primary recombination pathway.

Figure 2a shows the differential absorbance spectrum obtained 1.5 ps after excitation. This spectrum is broadly similar to those previously reported for atomically-thin MoS2 produced by other methods 3,15–

17,19,39,42,58,61,63–65. Bleach features are seen at spectral positions corresponding to the A and B excitons, as well as photo-induced absorption peaks red-shifted from both the A and B bleach, hence designated A* and B*, respectively. These spectral features have been attributed to a combination of effects by Pogna et al.16 Renormalization of both the bandgap and exciton binding energies leads to an overall redshift, simultaneously producing both the observed bleaches at A and B and the photo-induced absorption peaks, A* and B*; state filling also contributes to the bleach features. This model is supported by the results of other recent ultrafast studies of atomically-thin MoS2

19,39 and agrees with the data presented here. The NMP solvent shows no significant differential absorption signal as can be seen in figure S2.

The temporal evolution of each spectral feature for MoS2 in NMP is then shown in figure 2b. As shown in the inset to figure 2b, the B and B* transients reach their greatest magnitude around 1.3 ps after excitation whilst the A and A* features reach their maximum values more slowly in 1.7 and 2 ps respectively. Previous measurements of the rise-time have been pulse-width limited3,16,58,66, indicating that the initial response of MoS2 to photo-excitation occurs within ~100 fs or less, consistent with renormalization producing a significant part of that response16. In contrast, the state-filling component of the response grows as

the initially hot excitons cool to their ground state over a period of a few picoseconds21. In our experiment, the nonlinear response of the NMP has an influence on the detected signal for pump-probe delays up to 400 fs, as described in the Supporting Information, but this is not sufficient to explain the 1-2 ps rise times observed. Hence, these rise-times rather indicate that state-filling and trion formation, producing bleach and photo-induced absorption features respectively, have a larger effect than renormalization initially, although the contribution of the renormalisation may have been masked somewhat by the nonlinear response of the NMP. A reduced or absent contribution from renormalisation may be explained by a pre-existing population of free charges that is large enough that the photo-generation of additional charges by the pump pulse produces only a small fractional change in the overall carrier density, and thus has little renormalizing effect. In contrast, the exciton population in this material decays on a sub-ns timescale, as evidenced by the PL transients shown in figure 1d, so there is a negligible exciton population before the pump pulse arrives, allowing it to have strong state-filling and trion formation effects.

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Figure 3: Differential absorption spectra showing the spectral evolution of the A bleach feature and A* photo-induced absorption over (a) the initial 1.5 ps and (b) the subsequent 100 ps. The initial peak wavelengths of 669 nm and 694 nm of the A and A* features are shown as a dotted line as a guide to the eye.








Figure 3 shows the development of the spectrum over ~100 ps after initial excitation. In particular, the A bleach peak undergoes an initial red-shift over the first ~3 ps, followed by a blue-shift over the remaining 100 ps. In comparison, the A* feature does not shift significantly during the first 3 ps after excitation, but then undergoes a pronounced blue-shift coincident with this same decay timescale.

It is evident then that the evolution of the A and A* peaks are different from each other and to those at B and B*, indicating that another process is contributing to the differential absorbance spectrum in this region. The lack of redshift of the A* feature can be attributed to the overlap between the A and A* feature, such that any initial redshift is masked by the fast initial growth of the A bleach feature. However, the formation of trions could also produce an additional photo-induced absorption feature at the position of A*, overlapping somewhat with the A bleach14.

Trions are usually attributed to an intrinsic n-type doping in MoS2, but the longer rise time of the A* feature suggests that dissociated excitons also contribute to trion formation, with the additional processes of dissociation and electron capture which proceed trion formation leading to a slower rise time. This is in contrast to fast (< 500 fs) trion formation reported in sonication-produced MoS2 by Toskkou et al. who attribute the fast rise to the capture of photoexcited electrons in samples that were 10-20 monolayers in thickness and exhibited binding energies closer to the bulk value39. In our samples it is thus more likely that free

carriers are produced by the dissociation of photo-generated excitons as well as intrinsic doping.

As shown in figure 2b all four features decay initially from their maximum values with the same characteristic lifetime of 3.6 ± 0.2 ps. However, whilst the A, B and B* features reduce to less than 20% of the maximum magnitudes during this initial decay phase, the A* feature decreases to only about two-thirds of its maximum magnitude. The remainder for each feature then decays much more slowly; a global exponential fit to this longer-lived component yields a characteristic lifetime of 413 ± 37 ps, as shown in figure S3. The A* feature remains at a higher value than the other features to 1 ns, the limit of our experiment, indicating a longer-lived charge state than the A, B and B* features.

The form of the transient could be affected over the decay profile by the spectral shift seen in figure 3. However, as shown in figure S4, determining instead the peak maximum for each pump-probe delay yields a transient that is not significantly different than that obtained for a fixed probe wavelength. Additionally, the characteristic timescales of the initial redshift and subsequent blueshift are in agreement with the fast and slow timescales seen.

Decay transients with a similar form have been reported previously with the rapid initial decay attributed to exciton-exciton (XX) annihilation17, a process analogous to the Auger recombination of free charges, and the slower component due to single exciton or free electron decay channels such as defect-mediated recombination15.

The rate of XX annihilation is proportional to N X

2, where N X is the exciton density, resulting in a decay dynamic of the form 17:

N X ( t )=f XXN0

1+k A N0 t+f X e

−tτ (1)

where N 0 is the initial exciton density, k A is the XX annihilation rate constant, τ is the single exciton or free electron lifetime, and f XX and f X are the fractional contributions of XX annihilation and single exciton or free electron recombination, respectively. Figure 4 shows the absorbance transients for the A feature at different pump fluences, and

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Figure 4. Differential absorption transients for a probe wavelength of 694 nm for a range of fluences. The black lines are global fits to equation 1, yielding an annihilation rate constant kA = 4.4 ± 0.2 cm2/ps.








henceN 0, over the first 40 ps after excitation when XX annihilation dominates. The differential spectra for a range of pump fluences are also shown in figure S5. Equation 1 was fitted to the transients shown and describes the dynamic well in each case with an annihilation rate constant kA = 4.4 ± 0.2 cm2/ps, comparable to that found by Sun et al for a mechanically exfoliated sheet17. This model is consistent with the identification of the initial decay component with XX annihilation and the slower component with single exciton or free electron recombination.

Comparing this ultrasonicated MoS2

sample to the current literature, we can conclude that the main transient features observed are largely similar to those reported for MoS2 samples formed via other fabrication methods, of greater sheet thickness or size. The transient absorption features and the associated dynamics are consistent with the processes of band-gap renormalization, state-filling, and defect-assisted recombination identified previously3,10,15,16,20,61,18. The exception to this is the greater contribution from trion formation around the A* feature, leading to longer rise times and decay profiles, resulting in a larger absorption feature at long timescales.

Previous work has noted the lack of trion formation in monolayer films grown by CVD and attributed it to atmospheric adsorbates neutralising the intrinsic n-type behaviour of atomically-thin MoS2

19,20. The presence of a significant trion-related component to the ultrafast charge dynamics indicates that the MoS2 nanosheets investigated in this study have, in contrast, maintained their n-type nature.

4. SUMMARY AND CONCLUSION

Sub-nanosecond exciton and trion dynamics in a liquid dispersion of monolayer MoS2 nanosheets has been studied using ultrafast transient absorption spectroscopy. Photo-excitation of the samples with 100 fs pump pulses produced both a bleach feature and a red-shifted pump-induced absorption corresponding to each of the A and B excitons. For the B exciton, the growth and decay of these features are attributed predominantly to the formation and then mutual annihilation of excitons, which occur

over a few picoseconds. These effects also contribute to the behaviour of the features in the region of the A exciton. However, there is in addition a significant pump-induced absorption produced in this region caused by the formation of negative trions, which requires a significant population of free electrons. The slower growth relative to the exciton population causes the red-shift and then blue-shift of A exciton features in the differential absorption spectra.

We conclude that the ultrafast charge dynamics in MoS2 nanosheets dispersions produced by the ultrasonication of bulk material in NMP are broadly similar to those found in atomically-thin MoS2 produced by other means, such as mechanical exfoliation and CVD, and to ultrasonicated MoS2

deposited on a substrate. However, we see a difference from previous studies in that the synthesis and handling procedures used here preserve the intrinsic n-type doping of the MoS2 allowing the formation of trions to a significant, but not dominant, degree.

ASSOCIATED CONTENTSupporting informationAdditional figures and information are featured in the supporting information, including approximations in calculations, differential response of NMP, fluence dependent transient absorption spectra, as well as additional notes regarding data analysis. This is available as supplementary information accompanying this publication (PDF), and from the University of Manchester repository DOI:10.15127/1.302165.

AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected]

NotesThe authors declare no competing financial interest.

ACKNOWLEDGEMENTSThis work was funded by the EPSRC under grants EP/K008544/1, and EP/L01548X/1. NS and POB also thank the Parker family for funding.

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